1. Field of the Invention
The present invention relates to an apparatus according for growing thin films on a surface of a substrate. More particularly, the present invention relates to an apparatus for producing thin films on the surface of a substrate by subjecting the substrate to alternately repeated surface reactions of vapor-phase reactants.
2. Description of the Related Art and Summary of the Invention
Conventionally, thin-films are grown using vacuum evaporation deposition, the Molecular Beam Epitaxy (MBE) and other similar vacuum deposition methods, different variants of the Chemical Vapor Deposition (CVD) method (including low-pressure and organometallic CVD and plasma-enhanced CVD) or a deposition method of alternately repeated surface reactions called the Atomic Layer Epitaxy (ALE) method or Atomic Layer Deposition (ALD).
In MBE and CVD methods, the thin film growth rate is determined by the concentrations of the provided starting material in addition to other process variable. To achieve a uniform thickness of the layers deposited by these methods, the concentrations and reactivities of starting materials must be carefully kept constant on different surface areas of the substrate. If the different starting materials are allowed to mix with each other prior to reaching the substrate surface, as is the case in the CVD method, for instance, a chance of their mutual reaction arises. Then, a risk of microparticle formation already within the infeed channels of the gaseous reactants is imminent. Such microparticles generally have a deteriorating effect on the quality of the deposited thin film. Therefore, the possibility of premature reactions in MBE and CVD reactors, for instance, is avoided by heating the starting materials not earlier than at the substrate surfaces. In addition to heating, the desired reaction can be initiated using, e.g., a plasma discharge or other similar activating means.
In the MBE and CVD processes, the growth of thin films is primarily adjusted by controlling the infeed rates of starting materials impinging on the substrate. In contrast, the growth rate in the ALE process is controlled by the substrate surface qualities, rather than the starting material concentrations or flow variables. The only prerequisite in the ALE process is that the starting material is available in sufficient concentration to saturate the surface of the substrate. The ALE method is described, e.g., in FI patent publications 52,359 and 57,975 and in U.S. Pat. Nos. 4,058,430 and 4,389,973. Furthermore, equipment constructions suited to implement this method are disclosed in patent publications U.S. Pat. No. 5,855,680 and FI 100,409. Apparatuses for growing thin films are also described in the following publications: Material Science Report 4(7) (1989), p. 261, and Tyhjixc3x6tekniikka (Finnish publication for vacuum techniques), ISBN 951-794-422-5, pp. 253-261. These references are incorporated herein by reference.
In the ALE growth method described in FI Pat. No. 57,975, the reactant atoms or molecules are arranged to sweep over the substrates, thus impinging on their surface until a fully saturated molecular layer is formed thereon. Next, the excess reactant and the gaseous reaction products are removed from the substrates with the help of inert gas pulses passed over the substrates or, alternatively, by pumping the reaction space to a vacuum before the next gaseous pulse of a different reactant is admitted. The succession of the different gaseous reactant pulses and the diffusion barriers formed by the separating inert gas pulses or cycles of vacuum pumping result in a thin film growth controlled by the individual surface-chemical reactions of all these components. If necessary, the effect of the vacuum pumping cycle may be augmented by the inert gas flow. For the function of the process, it is typically irrelevant whether the gaseous reactants or the substrates are kept in motion; it only matters to keep the different reactants of the successive reactions separate from each other and to have them sweep successively over the substrate.
Most vacuum evaporators operate on the so-called xe2x80x9csingle-shotxe2x80x9d principle. In such an arrangement, a vaporized atom or molecule can impinge on the substrate only once. If no reaction with the substrate surface occurs, the atom/molecule rebounds or is revaporized so as to hit the apparatus walls or the vacuum pump, undergoing condensation therein. In hot-walled reactors, an atom or molecule that collides with the process chamber wall or the substrate can undergo revaporization and, hence, repeated impingements on the substrate. When applied to ALE process chambers, this xe2x80x9cmulti-shotxe2x80x9d principle can offer a number of benefits including improved efficiency of material consumption.
ALE reactions operating on the xe2x80x9cmulti-shotxe2x80x9d principle generally are designed for the use of a cassette unit in which a plurality of substrates can be taken simultaneously into the process chamber. In a modified arrangement, the substrates can be placed unmountedly into the process space formed by a pressure vessel, whereby the process space also serves as the reaction chamber wherein the vapor-phase reactants are reacted with the substrate surface in order to grow thin film structures. If a cassette unit designed for holding several substrates is employed, the reaction chamber is formed in the interior of the cassette unit. Use of a cassette unit shortens the growth time per substrate in respect to single-substrate cycling, whereby a higher production throughput is attained. Furthermore, a cassette unit arranged to be movable into and out from the process chamber can be dismantled and cleaned without interrupting the production flow because one cassette unit can be used in the process chamber while another one is being cleaned.
Batch processing is preferred in conventional ALE thin film processes because of the relatively slow production pace of the ALE method relative to other thin film growth techniques. The overall growth time per substrate of a thin film structure can be reduced in a batch process to a more competitive level. For the same reason, larger substrate sizes are also preferred.
In the deposition of thin films, the goal is to keep the process chambers continually running under controlled process conditions as to the temperature, pressure and other process parameters so that particulate matter of the ambient air and other chemical impurities cannot reach the substrates. Additionally, this arrangement eliminates the heating/cooling cycles that impair the reliability of process chambers and are time-consuming. Generally, a separate loading chamber is employed that is continually kept under a vacuum and to which the reactors are connected. Substrate loading thereto and unloading therefrom is accomplished by taking both the process chamber and the loading chamber to a vacuum, after which a valve between both chambers is opened and a robotic arm adapted into the loading chamber removes the processed substrate and loads a new substrate into the process chamber. Subsequently, the valve is closed and the process may be started after the substrate and the process chamber have attained the proper process conditions. Next, the processed substrate is moved via another controllable valve from the loading chamber to an air lock pumped to a vacuum, after which the valve is closed. Subsequently, the air lock can be pressurized, whereby the substrate can be removed from the system via a third valve opening into the ambient space. The new substrate to be processed is taken in the same fashion via the loading chamber into the process chamber.
Currently, process apparatuses equipped with this type of a loading chamber are available for single substrates only and they are not suited for accommodating heavy substrate cassette units. Depending on the batch and substrate size, such cassette units may weigh up to 200 kg, whereby devices designed for their handling must have a sturdy construction. Moreover, the lubrication of bearings and other similar components of the transfer means is problematic, because the lubricant required herein may affect the structure of the thin film to be grown.
The large cassette units used in conventional ALE deposition processes are assembled outside the process apparatus, after which the process chamber is opened and the cassette units are transferred as assembled entities into the process chamber. In the process chamber, the cassette unit is heated typically for 1-4 hours, processed for 2-4 hours and cooled up to ten hours depending on the cassette unit size. Furthermore, the assembly/disassembly of the cassette unit is a time-consuming operation. The ratio of the processing time vs. the work time required for other operations becomes even more disadvantageous when thin films of extremely shallow thickness (e.g., in the range 1-50 nm) are to be grown and the growth period may take from one minute to a few minutes. Under these circumstances, a major portion of the overall process cycle time in regard to the actual thin film growth period is lost in heating/cooling the reaction chamber structures, pressurizing the reactor, disassembling and reassembling the reaction chamber, pumping to a vacuum and reheating the system.
It is therefore an object of the present invention to provide an novel type of ALE apparatus that reduces the amount of time lost in heating/cooling the reaction chamber structures, pressurizing the reactor, disassembling and reassembling the reaction chamber, pumping to a vacuum and reheating the system.
Accordingly, one aspect of the invention involves equipping the process chamber with a separate loading chamber that can be pressurized independently from the process chamber so that the loading of the cassette unit into the process chamber can be carried out under a vacuum or a low-pressure inert gas atmosphere. The loading chamber can be complemented with preheating/cooling stations to shorten the overall processing cycle time. In a modified arrangement, a plurality of process chambers can be connected to each loading chamber. For moving the cassette unit, the reactor is provided with a transfer mechanism capable of accurately and sealably placing the cassette unit into its proper position in the process chamber and removing the same therefrom.
More specifically, the invention relates an apparatus that comprises at least one process chamber having a tightly sealable construction, at least one into the interior of said process chamber adaptable reaction chamber including a reaction space of which at least a portion is movable, infeed means connected to the reaction space for feeding reactants into the reaction space and outfeed means connected to the reaction space for discharging excess reactants and reaction gases from the reaction space, and at least one substrate adapted into said reaction space. The apparatus further includes at least one loading chamber in which the reaction space can be moved into and away from the process chamber and whose operating pressure can be controlled independently from said process chamber.
The invention offers significant benefits. For example, with the help of the loading chamber, the cassette unit can be moved into the process chamber and out therefrom so that the process chamber is at all times kept under stabilized process conditions. Hence, the steps of heating, pressurizing and pumping to a vacuum need not be carried out for the entire process chamber, but instead, for the substrates only, thus improving the efficiency of process chamber vastly. Owing to the use of the loading chamber, the interior parts of the process chamber are isolated from a direct connection to the ambient air, whereby the number of detrimental particles in the process chamber is reduced. The transfer mechanism employed in the embodiment of the invention is capable of moving relatively heavy cassette unit constructions and locating them accurately in a desired position within the process chamber. In another modified arrangement, a single loading chamber can be connected to a plurality of process chambers adapted to produce different kinds of thin film structures so that a plurality of thin-film layers can be grown without the need for intermediate transfer of the cassette units to ambient air atmosphere. This reduces the risk of possible contamination and the required number of thermal cycles.
It should be noted that certain objects and advantages of the invention have been described above for the purpose of describing the invention and the advantages achieved over the prior art. Of course, it is to be understood that not necessarily all such objects or advantages may be achieved in accordance with any particular embodiment of the invention. Thus, for example, those skilled in the art will recognize that the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein.